Plain English Meaning
Unlocking the spinning wheel inside a gyro instrument so it can move freely again and show correct readings.
Full Definition
The act of releasing a gyroscopic instrument's gyro from a locked, fixed position so that it can spin freely and respond to aircraft movement. Many gyroscopic instruments, such as attitude indicators and directional gyros, can be 'caged' (locked in a centered position) to protect the gyro during taxi, aerobatics, or unusual attitudes; uncaging frees the gyro to resume normal sensing and indicating.
Context Anchor
Seen with gyro flight instruments that can be locked for protection, starting, or resetting before normal use.
Derivation
From 'cage,' meaning to lock or restrain something in a fixed enclosure. Caging holds the gyro still; uncaging lets it loose. The word makes the action easy to picture: the gyro is 'let of its cage' to do its job.